Re-count available PV entries after reclaiming a PV chunk.

The call to reclaim_pv_chunk() in reserve_pv_entries() may free a
PV chunk with free entries belonging to the current pmap. In this
case we must account for the free entries that were reclaimed, or
reserve_pv_entries() may return without having reserved the requested
number of entries.

Reviewed by: alc, kib
Tested by: pho (previous version)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D15911